Electromigration is the transport of material caused by the gradual movement of ions in a conductor due to the momentum transfer between conducting electrons and diffusing metal atoms. The effect of electromigration is an important consideration to take into account in applications where high direct current densities are used, such as in microelectronics and related structures. In fact, electromigration is known to decrease the reliability of integrated circuits (ICs) and hence lead to a malfunction of the circuit. In the worst case, for example, electromigration leads to the eventual loss of one or more connections and intermittent failure of the entire circuit.
The effect of electromigration becomes an increasing concern as the size of the IC decreases. That is, as the structure size in ICs decreases, the practical significance of this effect increases. Thus, with increasing miniaturization the probability of failure due to electromigration increases in VLSI and ULSI circuits because both the power density and the current density increase.
In advanced semiconductor manufacturing processes, copper is used as the interconnect material. Basically, copper is preferred for its superior conductivity and electromigration resistance, for example, as compared to the prior Al(Cu) alloy interconnects. However, even copper interconnects have been facing the limitation of the electromigration lifetime or the upper limitation of the current density because of electromigration failure. To solve this problem, though, capping copper interconnects with CoWP or CoWB has been widely investigated. It has been found that the copper interconnects can have higher electromigration resistance when capped with CoWP or CoWB. This is because the top surface of the copper interconnects, which heretofore was capped with a dielectric barrier such as SiN or SiCN, and which resulted in this top surface being the fast diffusion path for copper electromigration, is effectively shut down for fast copper diffusivity and electromigration by a metal cap.
However, the CoWP or CoWB and even other metal capping materials can pose issues to the IC. For example, during the deposition of the CoWP or CoWB or other metal capping materials, residual metal can be deposited on the surface of the dielectric insulator material between the copper interconnects. Since the distance between the neighboring copper lines is becoming smaller as the device shrinks, the potential for shorting between the interconnects is becoming even more serious, especially if residual metal remains on the dielectric insulator material between the interconnects.
Accordingly, there exists a need in the art to overcome the deficiencies and limitations described hereinabove.